As an exhaust gas purification system for purifying NOx in an exhaust gas from a diesel engine, an SCR system using an SCR (selective catalytic reduction) device has been developed.
The SCR system supplies urea solution to an exhaust gas upstream of the SCR device to generate ammonia using heat of the exhaust gas, and using the ammonia, reduces NOx on an SCR catalyst to purify NOx (see, for example, patent literature 1).
In an SCR system, injection of urea solution is controlled according to the NOx concentration in an exhaust gas. For the control, a NOx sensor is provided in an exhaust pipe.
In the NOx sensor, after removal of O2 in an exhaust gas, NOx in the exhaust gas from which O2 has been removed is decomposed into N2 and O2, and an O2 concentration in the exhaust gas in which NOx has been decomposed into N2 and O2 is detected, whereby a NOx concentration is detected.